Within the framework of the present invention, it is considered that a time constraint is a constraint which requires a given arrival time, of RTA (“Required Time of Arrival”) type, at a particular waypoint of the flight trajectory followed by the aircraft.
Although not exclusively, the present invention can be applied in particular to the guidance of aircraft, such as in particular civilian transport airplanes, during an airport landing phase. It is known that such a landing phase is generally monitored and managed by air traffic controllers. One of the tasks of an air traffic controller is to ensure compliance, at a particular point of convergence in space, with a sequence of arrival times for the various aircraft converging towards this particular point, onward of which the final part of the landing is carried out.
A device for aiding the approach, with a view to a landing, is known from document FR-2 888 636. This device is mounted on an aircraft and uses a standard flight management system and standard guidance system.
During a flight requiring compliance with a time constraint, the following operations are generally carried out:    a) at least one time constraint of RTA type, which relates to a required arrival time at a particular waypoint, is entered into an onboard flight management system by an operator;    b) the values of parameters relating to the flight of the aircraft, such as the wind or the temperature for example, are determined;    c) guidance setpoints comprising speed setpoints which make it possible to guide the aircraft so that it arrives at said particular waypoint at said required arrival time, generally by following the flight trajectory initially envisaged, are determined with the aid of said values and of said time constraint; and    d) these guidance setpoints are applied to the aircraft.
The entry of an RTA time constraint relating to a particular waypoint into a flight management system, of FMS (“Flight Management System”) type for example, which makes it possible to fly the aircraft along a trajectory which is generally optimized in terms of parameters of the aircraft, exhibits drawbacks.
In particular, the flight management system does not anticipate the fact that the parameters (for example the wind, the temperature or the position of the aircraft) taken into account in the calculation of the guidance setpoints may be very unreliable. These parameters which are generally entered into the flight management system by an operator may be significantly different from the real parameters which will be encountered by the aircraft from its current position up to said waypoint. In particular, as regards wind and temperature, the difference between the values entered and the real values may be fairly considerable, mainly because of the poor accuracy of the weather data relating to wind and temperature, which are transmitted to the crew.
Furthermore, in the aforementioned step c), said speed setpoints are determined by taking account at least of an upper limit for the speed, which illustrates the maximum speed at which the aircraft is authorized to fly along the flight trajectory. So, to satisfy an entered time constraint, the flight management system determines, as speed setpoints, the speeds necessary in order to reach said particular waypoint at said given arrival time, without overstepping said upper speed limit (or maximum speed). Nevertheless, if necessary, the flight management system may be led to determine speed setpoints which correspond to said upper limit. Now, in such a situation, as the aircraft then follows the speed setpoints at its authorized maximum speed, it is impossible for it to accelerate further, should this prove to be necessary in the course of the flight. Thus, for example, if a headwind encountered by the aircraft is greater than the wind value used in the predictions, in particular because of an error in estimating the wind or because of the appearance of an unexpected headwind, so that an acceleration of the aircraft is necessary in order to be able to comply with the time constraint, said aircraft is not able to fly faster (since it is already flying at the maximum speed), and it is therefore not able to comply with the speed constraint.
In the same manner, in the aforementioned step c), said speed setpoints can be determined by also taking account of a lower limit for the speed, which illustrates the minimum speed at which the aircraft is authorized to fly along the flight trajectory. So, to satisfy an entered time constraint, the flight management system determines, as speed setpoints, the speeds necessary to reach said particular waypoint at said given arrival time, without overstepping said lower speed limit (or minimum speed). Nevertheless, if necessary, the management system may be led to determine speed setpoints which correspond to said lower limit. Now, in such a situation, as the aircraft then follows the speed setpoints at its authorized minimum speed, it is impossible for it to decelerate further, should this prove to be necessary in the course of the flight. Thus, for example, if a tailwind encountered by the aircraft is greater than the wind value used in the predictions, in particular because of an error in estimating the wind or because of the appearance of an unexpected tailwind, so that a deceleration of the aircraft is necessary in order to be able to comply with the time constraint, said aircraft is not able to fly slower (since it is already flying at the minimum speed), and it is therefore not able to comply with the speed constraint.
The aforementioned situations may be encountered, not only in the event of an error in the wind value used, but also in the event of an error in the temperature value used or in the event of an aircraft guidance error when the speed at which the aircraft is actually flying is different from the predicted speed used in the calculations.
The aforementioned standard guidance of an aircraft, with the aim of complying with a time constraint, is therefore not completely satisfactory.